1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus. More specifically, the present invention relates to the optical scanning apparatus for scanning a target surface with light, and relates to the image forming apparatus provided with such an optical scanning apparatus.
2. Description of the Related Art
Recently, a color and high speed processing is further required for an electrophotographic image forming apparatus such as a laser printer, a laser plotter, a digital copier, a plain paper facsimile and a combination thereof including MFP (MultiFunction Peripheral). Therefore, a tandem-type image forming apparatus having a plurality of photosensitive drums (usually, four drums) as image carrying bodies is often used.
In the tandem-type image forming apparatus having four photosensitive drums which are disposed in tandem, for example, a latent image is formed on each photosensitive drum by means of the optical scanning apparatus, after each photosensitive drum is charged by a charger. Then, the latent image on each photosensitive drum is developed into component color image by means of respective color developer different from each other (e.g. Yellow toner, Magenta toner, Cyan toner and Black toner) at each developing unit. The developed component images of respective colors are transferred onto a recording medium (e.g. paper) or an intermediate transfer belt so that each component color image (e.g. toner image) is superimposed one after another. Thus, a color image is formed.
In such an image forming apparatus, by the way, if an optical output from a light source changes, a color density or image density of an output image may change. In order to address this problem, the optical scanning apparatus usually performs an APC (Auto Power Control) for controlling an output level of the light source while monitoring a part of light beam ejected from the light source by a detector such as a photo diode. Hereinafter, the part of the light beam for monitoring may be also called monitoring light beam. The detector receives the monitoring light beam to monitor the output level of the light source.
In a case that reflected light from a polygon mirror returns to the light source while performing the APC, the output level of the light source cannot be accurately controlled.
For example, Japanese Patent No. 4428920 discloses an optical scanning apparatus, in which a plurality of light beams emitted from a plurality of light source means enter different deflection surfaces. The angles formed by the light beams emitted from the plurality of light source means and entering different deflection surfaces of a polygon mirror are parallel to each other in a main scanning cross section. The light beams entering the different deflection surfaces of the polygon mirror enter the polygon mirror in the same direction in the main scanning cross section. Light amount of each of the plurality of light source means is adjusted using the light beam reflected and deflected by the deflection surface before start of image formation or after the image formation. In the light amount adjustment in each light source means, when the light amount of arbitrary light source means is adjusted, other light source means are turned off.
Japanese Patent Application Laid-open No. 2007-025165 discloses an optical scanning apparatus provided with a light source, deflection means having a plurality of stages of multiple-reflection mirrors and a common rotation shaft, light beam splitting means splitting a beam from a common light source and making the split beams enter different stages of the multiple-reflection mirrors of the deflection means, a scanning optical system guiding the beam scanned by the deflection beams to a surface to be scanned, and light receiving means detecting the beam scanned by the deflection means. In the optical scanning apparatus, the split beams from the common light source scan different surfaces to be scanned.
Japanese Patent Application Laid-open No. 2008-257169 discloses an optical scanning apparatus provided with light beam splitting means provided between a light source and deflection means. The light beam splitting means splits a beam from a light source and makes the split beams enter toward deflection means with a phase difference of approximately π/2. In the optical scanning apparatus, the deflection means has four reflecting surfaces.
In spite of technologies disclosed by the aforementioned documents, there is a need to address the problem of the reflected return light without increasing a size of the apparatus or without increasing costs.